It is known that the human body""s healing response to injury typically includes the formation of what is commonly called scar tissue. A microscopically similar response also occurs within the transplanted vascular tissue of a person following transplantation of the saphenous vein into the coronary artery or peripheral vascular circulation to serve as a bypass.
One area of the vascular system of particular concern with respect to such injuries is saphenous veins that are used to provide bypass conduits for obstructed coronary arteries and also for obstructed peripheral arteries. Partial and even complete blockage of saphenous vein bypass grafts by the sequential and overlapping processes of thrombosis (formation of blood clots), fibrointimal hyperplasia (smooth muscle cell overgrowth) or formation of an atherosclerotic plaque upon the inner lining of the already thickened saphenous vein segment is a well known and frequent medical problem following coronary artery bypass grafting.
Occlusion of coronary artery bypass grafts occurs in more than fifty percent of saphenous vein grafts by the time the graft is ten years old, affecting the majority of patients with saphenous vein bypass grafts.
In conventional treatment, such blockages may be treated using atherectomy devices, which mechanically remove the plaque; hot or cold lasers, which vaporize the plaque; stents, which hold the artery open; and other devices and procedures which have the object of allowing increased blood flow through the bypass conduit. The most common such procedure is the percutaneous transluminal coronary angioplasty (PTCA) procedure, more commonly referred to as balloon angioplasty. In this procedure, a catheter having an inflatable balloon at its distal end is introduced into the coronary artery or saphenous vein bypass graft, the uninflated balloon is positioned at the stenotic site and the balloon is inflated. Inflation of the balloon disrupts and flattens the plaque against the vessel wall, and stretches the arterial or venous wall, resulting in enlargement of the intraluminal passageway and increased blood flow. After such expansion, the balloon is deflated and the balloon catheter removed.
However, all of the above conventional remedies for vascular occlusion are performed after the vein is sewn into the coronary artery system and after the problem of vascular occlusive disease has developed and become a problem for the patient. By contrast, the present invention provides methods and devices to prevent such blockage from occurring, by irradiation of the bypass graft during the bypass graft surgical procedure.
Smooth muscle cell migration and proliferation are stimulated in several ways during transplant of vascular tissue such as saphenous veins, including mechanical trauma, and baurotrama. Such stimulation also occurs from denudation of the endothelium, and from mitogenic proliferative factors, such as platelet-derived growth factor, fibroblast growth factors, and epidermal growth factor. These influences initiate the body""s own natural repair and healing process. During this healing process, vascular smooth muscle cells migrate into the intima and prolferate. The formation of scar tissue by smooth muscle proliferation, also known as fibrointimal hyperplasia, is believed to be a major contributor to the occlusion of saphenous vein bypass grafts following placement of vein grafts into the aortocoronary circulation.
Prior efforts to inhibit occlusion of saphenous vein grafts have included optimal antiplatelet therapy with the combination of aspirin and dipyridamole, which reduced the rate of occlusion from about 25% at one year to about 11%. However, the improvement relating to the prevention of thrombosis had no identifiable beneficial effect upon the process of fibrointimal proliferation. Fibrointimal proliferation results in about 25% lumenal narrowing by the end of one year in all vein graft segments.
Although radiation therapy has shown promise, particularly in inhibiting neotintimal hyperplasia within the in vivo arterial circulation, the devices available for delivery of radiation sources have been limited to treating a segment of vascular tissue within the patient, and have not been applied to treatment of vascular tissue being transplanted from one site of the body to another while it is outside of the patient""s body.
The present invention includes ex vivo methods of treating vascular tissue, e.g., saphenous vein coronary artery bypass grafts, with endovascular irradiation, particularly beta irradiation. This method is particularly suitable for ex vivo applications. The inventor has also developed devices suitable for such methods, including sterile sleeves for endovascular positioning of the radiation source, housings for the radiation sources, and a radiation seed safe module for the purpose of shielding and storing the radiation source, said module containing radiation pellets for insertion into the sleeve lumen or cavity.
The methods and devices of the present invention are suitable for reducing fibrointimal proliferation or neointimal hyperplasia, vascular lesions that commonly occur in the treatment of cardiovascular disease, e.g. balloon angioplasty, coronary bypass surgery. The present invention is also suitable for achieving a clinically significant decrease in the morbidity and mortality resulting from SVG occlusions, particularly in view of the large number of patients at risk. About 220,000 patients undergo each year coronary artery bypass surgery with a saphenous vein as the bypass conduit, of which about 22,000 would substantially benefit from the methods and devices of the present invention.
The present invention is directed to devices and methods for delivering one or more treating elements, such as a radiation source, into or outside of a suitable sleeve or elongated means, upon which is placed a segment of a suitable graft, e.g., saphenouse vein. The vein has been removed from the patient""s leg or arm and divided into one or more tubular segments for use as bypass material. The vein is mounted on the sleeve or elongated means. It is then subjected to radiation effective to reduce or inhibit overgrowth of vascular repair tissue, by exposure to a radiation source within the lumen of the graft or outside of the graft. Having irradiated the graft, it is then implanted back into the patient before finishing the bypass surgery.
Methods of irradiating saphenous coronary bypass conduits is also disclosed, including a suitable apparatus for ex vivo applications. One preferred method and apparatus of the present invention involves coronary bypass surgery with an autologous saphenous vein graft, using 90Sr seeds as a beta radiation source.